In the existing preparation process of semiconductor devices, chips are processed in batch, and a large number of complicated devices are formed in one and the same wafer. Recently, with rapid development of the very large scale integration (VLSI), the chips are developed toward high integration density and miniaturization. In the preparation process, the critical dimensions (CD) of chip are further reduced as well, which raises a higher requirement to a lithography process. However, due to the restriction by the light source wavelength of scanner, the existing immersion scanner with 193-nanometer (nm) wavelength cannot meet requirements of a process below 32 nm. In order to satisfy the requirements of the process below 32 nm, methods mainly used in practice are double patterning technique and extreme ultraviolet (EUV) Lithography technique.
Among these techniques, the lithography technique with spacer-defined double patterning is one of the double patterning techniques. The spacer-defined double patterning has been widely used in preparation processes of trench structures, for example, in pattern definition of the trench structures such as a trench structure in an active region, a metal trench structure, etc. As shown in FIGS. 1A to 1H, by taking the metal trench structure as an example, the above-mentioned spacer-defined double patterning process applies into the preparation process of trench structure mainly by the following processes: first, depositing in sequence a lower layer medium 12, an interlayer dielectric layer (inter metal dielectric) 13 and a sacrificial hard mask layer 14 on a substrate 11, so as to form a trench structure in the interlayer dielectric layer 13, in which the trench will eventually stop on the lower layer medium 12; next, coating a layer of photoresist 15 on the sacrificial hard mask layer 14 (as shown in FIG. 1B), and performing a lithography process (exposure, development) so as to form photoresist retention structures 15a, 15b (as shown in FIG. 1C); and then etching the sacrificial hard mask layer 14 by using the photoresist retention structures 15a, 15b as masks to form retention structures 14a, 14b of the sacrificial hard mask layer, and removing the photoresist retention structures 15a, 15b (as shown in FIG. 1D); next, depositing a layer of silicon nitride film 16 for forming a spacer on the retention structures 14a, 14b of the hard mask layer (as shown in FIG. 1E), etching the silicon nitride film 16 with an anisotropic dry etching process to form spacers 16a, 16b, 16c and 16d on both sides of the retention structures 14a, 14b of the sacrificial hard mask layer respectively (as shown in FIG. 1F), and then removing the retention structures 14a, 14b of the sacrificial hard mask layer (as shown in FIG. 1G) and performing a loop trimming exposure, and etching the interlayer dielectric layer 13 by using the silicon nitride spacers 16a, 16b, 16c and 16d and the loop trimming photoresist 17 as masks to form desired lithography patterns of trench (as shown in FIG. 1H); and eventually, removing the silicon nitride spacer masks and the loop trimming photoresist 17 to form the final pattern of the metal trench (as shown in FIG. 1I). As spacers are located on both sides of the retention structures of the sacrificial hard mask layer after the formation of the spacers, the pattern density of the spacers would be twice of the pattern density originally formed from the photoresist, and the pitch of the lithography pattern would be reduced to one half of the original pitch correspondingly, so that smaller critical dimensions can be obtained.
In the process of lithography pattern definition of trenches by means of spacer-defined double patterning technique, a minimum line width of the retention structure of the sacrificial hard mask layer (see “b” in FIG. 1D) is determined by the lithography process (see “a” in FIG. 1C), so that the minimum width “c” of the trench in the final pattern (as shown in FIG. 1I) is also determined by the lithography process. Moreover, the minimum width that can be exposed by the photoresist is in turn determined by the factors such as the light source wavelength and the thickness of the photoresist and so on. In the case where the conditions such as the light source wavelength and the thickness of the photoresist are determined, the minimum line width that can be achieved by the lithography process is determined correspondingly.